The cloned ompA gene from Serratia marcescens was fully expressed in Escherichia coli and its product correctly assembled into the outer membrane. The S. marcescens polypeptide was not functionally equivalent to the E. coli OmpA protein, which serves as a phage receptor and as a component of several colicin uptake systems. DNA sequence analysis of the gene showed that three regions of the protein likely to be exposed on the cell surface not only differed extensively from the corresponding regions of the E. coli polypeptide but also from all other sequenced OmpA proteins. It is suggested that this sequence polymorphism represents a safety mechanism by which the various enterobacterial species can avoid cross-infection by noxious agents such as phages or colicins.